The electronic technology has recently made remarkable progress and has been successful in the reduction in size and weight of various devices. Combined with the miniaturization and weight reduction of electronic devices, batteries that supply power to such devices are also required to be smaller and lighter. Nonaqueous electrolyte lithium secondary batteries with small volume and lightweight can supply large amounts of energy, and are used as power sources mainly in small household electronic devices such as cellular phones, personal computers and video camcorders.
Electrodes (positive and negative electrodes) of the nonaqueous electrolyte secondary batteries are prepared by, for example, mixing powdery electrode-forming materials such as electrode active materials and optional conductive auxiliaries with a binder (a binding agent), dissolving or dispersing the mixture in an appropriate solvent, and applying the resultant electrode mixture on collectors followed by drying to form mixture layers.
The binders should be resistant to nonaqueous electrolyte solutions obtained by dissolving electrolytes such as LiPF6 and LiClO4 in nonaqueous solvents such as ethylene carbonate and propylene carbonate. They are also required to have low specific resistance and good membrane-forming properties. In detail, vinylidene fluoride polymers are generally used as binders.
Patent Literature 1 discloses vinylidene fluoride copolymers obtained by copolymerizing vinylidene fluoride with unsaturated dibasic acid monoesters. Patent Literature 1 is directed to the provision of vinylidene fluoride polymers that show good adhesion with substrates such as metals, have excellent chemical resistance and are producible by aqueous polymerization. In the literature, electrode mixtures are described in which the polymers are used as binders for battery electrode production. However, the literature does not specify any particular components other than the polymers that are contained in the electrode mixtures.
Low peel strength in electrodes, in detail between a mixture layer and a collector, has caused problems that the electrodes are cracked or separated during steps such as pressing, slitting and winding. Such problems can lead to low battery performance, and separated pieces can penetrate the separator to cause short circuits. The problems are therefore important items to be controlled in the electrode production.
Patent Literature 2 discloses a process of producing battery electrodes having excellent peel strength between a collector and a mixture layer, wherein an acid is added to a slurry that will be applied to the collector. Patent Literature 2 describes that the acids are preferably organic acids, and more preferably carboxylic acids.
However, the peel strength between the collector and the mixture layer is still insufficient. Further improvements have therefore been required.
Citation List
Patent Literatures
    Patent Literature 1: JP-A-H06-172452    Patent Literature 2: JP-A-H02-68855